Transcript PP Ch 2x
Chapter 2 Mechanical Equilibrium An object in mechanical equilibrium is stable, without changes in motion. Chapter 2- your syllabus should be in your binder and labeled as Day 1. Take out a sheet of paper label it Day 2. Copy down the Objective and Warm-Up. Then answer the Warm-Up. You have 5 min. Objective • Differentiate between force and net force • Define mechanical equilibrium • Introduce vectors Warm-Up • Name a situation that has applied force to your body. • Buddy Check 2.1 Force Force vs Net Force Newtons= units for force How do we stand up? Weight is caused by gravity. Vector vs Scalar • Vectors – Have direction and magnitude • Ex: force and several others that we will learn about during this course. • Scalar – Magnitude only • Ex: time, area, volume A stretched spring is under a “stretching force” called tension. Mechanical Equilibrium Static • Object at rest Dynamic- push force = friction • Object moving at constant speed in a straight line – A car moving on a straight road at a constant speed of 45 mph. 1. 2. 3. 4. 5. When you hold a rock in your hand at rest, the forces on the rock a. are mainly due to gravity. b. are mainly due to the upward push of your hand. c. cancel to zero. d. don’t act unless the rock is dropped. Burl and Paul have combined weights of 1300 N. The tensions in the supporting ropes that support the scaffold they stand on add to 1700 N. The weight of the scaffold itself must be a. 400 N. b. 500 N. c. 600 N. d. 3000 N. Harry gives his little sister a piggyback ride. Harry weighs 400 N and his little sister weighs 200 N. The support force supplied by the floor must be a. 200 N. b. 400 N. c. 600 N. d. more than 600 N. When a desk is horizontally pushed across a floor at a steady speed in a straight-line direction, the amount of friction acting on the desk is a. less than the pushing force. b. equal to the pushing force. c. greater than the pushing force. d. dependent on the speed of the sliding crate. When Nellie hangs at rest by a pair of ropes, the tensions in the ropes a. always equal her weight. b. always equal half her weight. c. depend on the angle of the ropes to the vertical. d. are twice her weight. Exit Ticket • What is the net force on a bathroom scale when a 110-pound person stands on it? • Suppose you stand on two bathroom scales with your weight evenly distributed between the two scales. What is the reading on each of the scales? What happens when you stand with more of your weight on one foot than the other? Answer • Zero–the scale is at rest. The scale reads the support force, not the net force. • In the first case, the reading on each scale is half your weight. In the second case, if you lean more on one scale than the other, more than half your weight will be read on that scale but less than half on the other. The total support force adds up to your weight. Chapter 2 Objective • Differentiate between force and net force • Define mechanical equilibrium • Introduce vectors Warm-Up • If the gymnast hangs with her weight evenly divided between the two rings, how would scale readings in both supporting ropes compare with her weight? Suppose she hangs with slightly more of her weight supported by the left ring. How would a scale on the right read? Answer • In the first case, the reading on each scale will be half her weight. In the second case, when more of her weight is supported by the left ring, the reading on the right reduces to less than half her weight. The sum of the scale readings always equals her weight. 2.1-2.4 Book Questions- there are 15 total •2, 6, 7, 9, 10, 14, 15, 21, 23, 25, 26, 27, 28, 45, 46 Resultant Same direction- parallel • Vector 1 + Vector 2= resultant Nonparallel- Parallelogram Rule Vectors!! • Old boats had square sails. They could only sail downwind (or with the wind). They basically could only go as fast as the wind. Vectors!! • Sailing against the wind creates LIFT, which allows for more speed. The tension in the ropes at an angle is greater than half her weight Remember this? This little girl hangs on a rope that must have an upward VECTOR = to her downward force, as the angle of the rope increases tension on the rope also increases, or it can become unequal causing the rope to break. Exit Ticket • Two sets of swings are shown at right. If the children on the swings are of equal weights, the ropes of which swing are more likely to break? Answer • The tension is greater in the ropes hanging at an angle. The angled ropes are more likely to break than the vertical ropes. Chapter 2 Objective • Differentiate between force and net force • Define mechanical equilibrium • Introduce vectors Warm-Up An airplane flies horizontally at constant speed in a straightline direction. Its state of motion is unchanging. In other words, it is in equilibrium. Two horizontal forces act on the plane. One is the thrust of the propeller that pulls it forward. The other is the force of air resistance (air friction) that acts in the opposite direction. Which force is greater? Answer • Neither, for both forces have the same strength. Call the thrust positive. Then the air resistance is negative. Since the plane is in equilibrium, the two forces combine to equal zero. Exit Ticket • Consider what would happen if you suspended a 10-N object midway along a very tight, horizontally stretched guitar string. Is it possible for the string to remain horizontal without a slight sag at the point of suspension? Answer • No way! If the 10-N load is to hang in equilibrium, there must be a supporting 10-N upward resultant. The tension in each half of the guitar string must form a parallelogram with a vertically upward 10-N resultant. Chapter 7 Review- Test tomorrow Objective • Review Warm-Up • Answer T/F 1. An astronaut weighs the same on Earth as in space. 2. Inertia is the property that every material object has; inertia resists changes in an object's state of motion. 3. The reason a penny thrown straight up inside an airplane will come back to your hand is that you, the air inside the plane, and the penny are all moving at the same horizontal velocity. What did we learn? • • • • • • Vectors vs Scalar Mechanical Equilibrium (static v dynamic) Parallel Forces Non-Parallel Forces Scientist Inertia Exit Ticket 1. After a cannonball is fired into frictionless space, the amount of force needed to keep it going equals 2. Compared to its weight on Earth, a 10-kg object on the moon will weigh 3. The force required to maintain an object at a constant speed in free space is equal to 4. The mass of a sheep that weighs 210N is about 5. You would have the largest mass of gold if your chunk of gold weighed 1 N on (Jupiter, Earth, Saturn) Trashket Ball • You must stand where the tape is • To have the opportunity to play you must answer the questions correct. • In the future we will play in teams and keep score. Add to Quiz 11. Burl and Paul have a combined weights of 1300N. The tensions in the supporting ropes that support the scaffold they are standing on add to 1700N. Solve for the weight of the scaffold. Show Work 12. The idea that every force (weight) or horizontal push force has an equal but opposite force (support force or friction) is considered a law of nature. Is this a law of nature or a law of God, or both? Support your answer in 1-2 sentences